| Transition metal compounds is a main topic in condensed matter physics for their interesting and abnormal properties, such as high-Tc superconductivity, high thermoelectric efficiency, et al, which have attracted much attention of researching. In this dissertation, we have studied the electron-type superconductor Nd2-xCexCuO4 single crystal and thermoelectric materials Bi2-xPbxSr2Co2Oy and BixTiS2 polycrystalline in detail. The dissertation was arranged as six parts:Chapter 1 Summaries for thermoelectric effects and the properties of High-Tc superconductorsIn part I, the theories of resistivity, thermopower and thermal conductivity, are firstly summarized, and the physical insight of these transport properties is also discussed. And then, the applications of thermoelectric effects are recommended. In part II, the author reviews the structure and phase diagram of the high-TC superconductors. Their anomalous properties are briefly discussed and the recent development in theoretical and experimental physics about this topic is also mentioned.Chapter 2 A new method of fast thermopower measurementIn this chapter, the traditional steady-method of the measurement on thermopower and thermal conductivity is introduced at first. To quicken the measurement of thermopower, the new method of fast thermopower measurement was developed. In the same request of resolution, this method reduces the measurement time considerably, with a factor of five. Chapter 3 Studies on the transport properties of Nd1.85Ce0.15CuO4 single crystalIn this chapter, the transport properties of Nd1.85Ce0.15CuO4 under different annealed conditions are investigated systematically. The transport properties, such as resistivity, thermopower and thermal conductivity, can be well explained in a two-band model, with a hole-type narrow band and an electron-type broad band. The transition temperature of superconducting is not affected by carrier concentration, but by the Ce doping level. The main effect of annealing processes at 900℃is to remove excess oxygen. As a result, Fermi level increases. The electron-type broad band is not changed, while the hole-type narrow band becomes narrower, and its central energy is enhanced. Furthermore, the decomposition, produced when annealed at 1080℃, decreases Fermi level and the central energy of narrow band. The removal of oxygen enhances the anisotropy while the decomposition does opposite.Chapter 4 Resistivity and Thermopower of NdxCu1-xZnxO4 single crystalsIn this chapter, resistivity and thermopower of Nd2Cu1-xZnxO4(x=0, 0.01) single crystal are studied in detail, before and after annealed in N2 at 900℃. At high temperatures (>75 K), both resistivity and thermopower represent a semi-conducting behavior, while at intermediate temperatures (35 K～75 K), both of them behave as variable range hopping (VRH), indicating that Nd2Cu1-xZnxO4 is a Mott insulator with disorder. The gaps, calculated from thermopower are much smaller than that from resistivity, indicating the presence of polarons. The transition of magnetic structure would influence the behavior of thermopower remarkably, and affect the behavior of resistivity slightly. The antiferromagnetic transition around 235 K brings in the special thermal hysteresis in thermopower, and around 75 K, resistivity and thermopower transfer from semi-conducting behavior to VRH behavior, caused by magnetic transition. Furthermore, the magnetic transition around 35 K also influences the behavior of thermopower. Below 35 K, the thermopower tends to saturate before annealed, while decreases rapidly after annealed, whose origin still calls for more investigations.Chapter 5 Study on the thermoelectric properties of Bi2-xPbxSr2Co2OyIn this chapter, the thermopower and resistivity of Bi2-xPbxSr2Co2Oy polycrystalline samples with different Pb doping level are investigated in detail. The large constant behavior of thermopower at high temperatures, together with metallic resistivity, indicates a narrower band contribution. As a result of Anderson localization, brought in by disorder, the resistivity represents thermal-activated behavior at intermediate temperatures, while no change can be seen in the thermopower. At low temperatures, all the thermal-activated electrons and holes are in the localized states, thus both the resistivity and thermopower represent variable range hopping behavior. The doping of heavy metal element Pb enhances the thermoelectric efficiency.Chapter 6 Study on the thermoelectric properties of BixTiS2In this chapter, the thermoelectric properties of BixTiS2 polycrystalline samples, with different Bi intercalating level, are investigated in detail. The main effect of Bi intercalation is to transfer five electrons to 3d band of Ti atoms in the host layers, and increases the carrier concentration. While Bi intercalation does not bring in any new scattering mechanism to influence electron transport, thus the thermopower can be well normalized. The thermopower are related to the special lens-shaped Fermi pockets of TiS2 system. With more carriers are doped, the Fermi pockets expand, and the characteristic energy hωc and corresponding temperature of the phonon peak increase. Bi intercalation reduces the thermal conductivity of phonons, and thus enhances the thermoelectric efficiency. However, if resistivity, thermopower and thermal conductivity are taken into account together, the thermoelectric figure of merit ZT increases with increasing intercalation and then decreases. There is a best ZT when the Bi intercalation is about 0.15.
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